Spark plug center electrode with a split truncated head



1957 J. l. JALBING T 3,346,7 60

SPARK PLUG CENTER ELECTRODE WITH A SPLIT TRUNCATED HEAD Filed Aug. 30, 1965 IN VENTORS M2:

M02)? Z x2522; 45 BY 21965! 72? 227? United States Patent Ofiice 3,345,769 Patented Oct. 10, 1967 3,346,760 SPARK PLUG CENTER ELECTRQDE WITH A SPLIT TRUNCATED HEAD John I. .l'albing, Millington, and Ernest H. Myers, Flushing, Mich, assignors to General Motors Corporation,

Detroit, Mich, a corporation of Delaware Filed Aug. 30, 1965, Ser. No. 483,546 9 Claims. (Cl. 313-136) This invention relates to a spark plug, and more particularly to an improved center electrode.

It is well recognized that center electrodes are locked within a spark plug insulator centerbore by means of a conductive glass seal formed in situ about the inner end or head of the center electrode. Numerous spark plug failures result due to the inability of the seal to satisfactorily grip or lock the head of the center electrode for extended periods of time under service conditions. Electrode heads of various shapes have been devised to improve the locking relationship existing between the head of the center electrode and the seal.

It is a basic object of this invention to provide an improved spark plug in which the center electrode has superior locking ability. It is another object of this invention to provide spark plugs in which the center electrode head is resistant to rocking movement, circular movement, and vertical movement in the spark plug during service. It is yet another object of this invention to provide a spark plug in which the center electrode permits the glass seal to grip the center electrode head around the middle portion of the head more tightly. It is a further object of this invention to provide a spark plug in which the center electrode facilitates the flow of the glass seal charge about the head.

These and other objects are accomplished by a spark plug in which the center electrode has an enlarged head wherein the head has a reverse taper or an inverted frusto conical shape and a plurality of slots across the top thereof. Specifically, in the illustrated embodiment disclosed herein, the enlarged center electrode head comprises an inverted frusto conical lower portion and an upper portion. The upper portion has a reverse taper or frusto conical shape. Across the top of the upper portion is a cross slot where the slots or grooves are in a perpendicular direction to the longitudinal axis of the center electrode. The outer edge of the upper end of the upper portion of the enlarged head is in a touching or close proximity relationship to the walls of the insulator centerbore. When the spark plug assembly containing the insulator, center electrode and glass seal charge placed therein is heated to a temperature sutlicient to soften the glass, downward pressure is applied to the fluid glass forcing it through the slots or grooves to completely fill the space under the outer edge of the upper portion and between the walls of the insulator centerbore and the frusto conical sides of the upper portion of the center electrode head. The design of the center electrode head causes the fluid glass to flow underneath the outer edge of the upper end of the head to form a glass seal which tightly interlocks the center electrode head with the rest of the glass seal positioned above the center electrode head. This tight interlocking relationship existing between the head and the glass seal eliminates the problems caused by center electrodes which have broken loose from the glass seal.

Other objects and advantages of this invention will be more apparent from the following detailed description, reference being made to the accompanying drawings wherein a preferred embodiment of this invention is shown.

In the drawings:

FIGURE 1 is a side view of a spark plug having the improved center electrode partly in cross section and partly in elevation;

FIGURE 2 is an enlarged view of the center electrode in elevation;

FIGURE 3 is an enlarged end view of the center electrode head;

FIGURE 4 is a cross sectional View of the center electrode head of FIGURE 3 taken along the line 44.

Referring to FIGURE 1 of the drawings, the spark plug 8 comprises a conventional outer tubular metal shell 12 having a ground electrode 14 welded to the lower end thereof. Positioned within the metal shell 12 and secured in the conventional manner is an insulator 16. The insulator 16 is formed with a centerbore 17 consisting of a lower portion 18 of relatively small diameter and an upper portion 20 of larger diameter. A tapered ledge 21 connects lower portion 18 to upper portion 20. Positioned in the lower portion 18 of the insulator centerbore is the center electrode 22, the serrated lower end 28 thereof projecting beyond the lower tip of the insulator 16. The enlarged upper end of the center electrode 22 is the center electrodev head 24 as will be hereafter fully described. The center electrode head 24 has a tapered or inverted frusto conical shaped shoulder 26 which rests on the insulator centerbore ledge 21. A glass seal 30, as will be hereafter fully described, forms a hermetic seal between the center electrode head 24, the insulator center bore wall of the upper portion 29 and the lower end of the terminal 32. The upper end of the terminal 32 protrudes out of the top of the insulator center bore 20.

As shown in FIGURE 2, the center electrode 22 has a head 24 consisting of a tapered or inverted frusto conical shaped shoulder 26 and a top portion 34 having sides 36 which form a reverse taper or an inverted frusto conical shape, that is the sides 36 of the top portion 34 slant in an upward and outward direction from the axis of the center electrode 22. Head 24 has grooves or cross slots 38 and 40 cutting through the top portion 34 at right angles to each other and perpendicular to the longitudinal axis of the center electrode 22. Although the cross slots 38 in the preferred embodiment are perpendicular to the longitudinal axis of the center electrode, it is not necessary that these slots be perpendicular. The cross slots 38 and 40 are depressions or grooves which provide a passageway or connecting path between the glass above and below the outer edge 42 of the top portion 34.

FIGURE 3 is a top view of the head 24 which shows the cross slots 38 and 40 at right angles to each other. The outer edge 42 at the upper end of the top portion 34 extends outwardly almost as far as the outside edge of shoulder 26. The closeness of the outer edge 42 to the insulator centerbore walls, as shown in FIGURE 1, provide for maximum strength of the bond between the insulator walls and the head 24. The maximum strength of the bond would be obtained if the outer edge 42 actually touched the insulator centerbore walls. However, since this is not practical, the electrode head 24 is constructed to give the outer edge 42 a minimum amount of clearance with the insulator centerbore walls. The grooves or slots at 38 and 40, as shown in FIGURES 2 and 3, provide the primary passageway through which the molten glass may flow below the outer edge 42 and around the side walls 36 as will be hereinafter described.

As shown in FIGURE 4, the sides 36 of the top portion 34- form an angle of 15 to 20 with a vertical line drawn to the outer edge 42. This angle is measured at a slot opening. Although the angle of the reverse taper is 15 to 20 in the preferred embodiment, this angle may vary from 10 to The larger the angle of the reverse taper formed by the sides 36, the greater the gripping power. However, practical considerations in the heading process by which these heads are formed on the center electrodes restrict the angle to approximately 15 to 20. It has been found that angles less than do not afford sufiicient locking or gripping characteristics. As shown in FIGURE 4, the slot 4% forms an angle of 40 to 60 with the apex of the angle in the lower part of the top portion 34. Although this slot is in the form of a V- groove having an angle of 40 to 60, the shape of the slot is not critical, and this slot could be in the form of a rectangular slot, etc. It was observed that by using a two-stroke header to form the slot in the electrode head 24, that a V-groove slot having an angle of 40 to 60 yielded a head 24 having straight sides 36 which formed a reverse taper angle of to Slot angles greater than 60 made by the two-stroke header, tended to have the undesirable effect of making the side 36 convex or bulging. Slot angles less than did not yield a large enough angle for the reverse taper. The V-shaped form of slot was chosen in the preferred embodiment because it was the most practical in the heading apparatus used by the inventors.

Referring now to FIGURES 1 and 2, this invention will now be described in greater detail in terms of a method for making a spark plug. As shown in FIGURE 1, the center electrode 22 is inserted into the centerbore of the insulator body 16 so that the center electrode shoulder 26 rests on the insulator ledge 21. A metal-glass seal powder load 30 ranging from 0.25 to 0.35 gram is placed on top of the center electrode head 24 in the insulator centerbore upper portion 20 in the conventional manner and tamped. This invention is not restricted to the use of a metal-glass powder but is applicable to any sealing composition commonly used in this application. The composition of the metal-glass seal used in the preferred embodiment was glass, copper powder, 2% boric anhydride, and 3% hydrogenated cottonseed oil. The glass contains 65% SiO 23% B 0 5% A1 0 and 7% Na O. The mesh size of the glass powder does not appreciably affect the performance of a seal. A mesh size of 200 for the glass powder was found to be satisfactory. Boric anhydride is a flux frequently used to lower the melting point of spark plug glass seals. The amount of flux, boric anhydride in the preferred embodiment, in the metal-glass mixture 30 ranges from 1 to 4%. Hydrogenated cottonseed oil is a binder frequently used for spark plug glass seals which burns off during the firing step to prevent oxidation of the metal. The amount of binder, hydrogenated oil in the preferred embodiment, in the metal-glass mixture ranges from 1 to 3%. Copper is the metal most commonly used in the metal-glass seals although nickel, iron, tungsten, Kovar, etc. may be used. The terminal 32 is then inserted into the insulator centerbore 20. This assembly is then heated in a tunnel kiln to 1650 F. in fifteen minutes and held at that temperature for five minutes in order to melt the glass powder 30. The terminal wire 32 is pressed downward forcing the fluid glass 30 through the slots 33 and 40 primarily and a small amount around the outer edge 42 of the top portion 34 to fill the region under the top edge 42 and around the sides 36. Upon cooling a very strong interlocking bond between the center electrode head 24, the glass seal 30 and the insulator centerbore walls of portion 20 prevent the center electrode 22 from any movement in the spark plug insulator 16.

This center electrode head 24 is of such a design that a superior locking relationship exists between the glass, the center electrode head and the insulator. The combination of the reverse taper, the plurality of slots, and the close proximity or near touching of the outer edge 42 of the top portion 34 to the walls of the insulator combine to achieve these results. The reverse taper or inverted frusto conical shape is of extreme importance in obtaining a proper locking arrangement between the head and the glass seal. The reverse taper or inverted frusto conical shaped formed by sides 35 permits the glass to be underneath or below the outer edge 42 of the head top portion 34; thereby providing a good grip on the electrode head which prevents vertical movement of the center electrode.

The cross slots 38 and 4% of the preferred embodiment are essential in providing a passageway for the fluid glass 39 which would normally be unable to pass by the top edge 42 of the head top portion 34 in sufficient quantity to completely fill the area underneath the top edge 42 due to the close proximity or near touching of the top edge 42 to the insulator centerbore walls. These slots permit the glass to completely fill the area on the underside of the reverse taper sides 36 of the top portion. A single slot traversing the top of the top portion 34 would fill the underside of the reverse taper sides 36 in the area surrounding the ends of the slot. However, a single slot may not fill completely the area on the underside of the reverse taper sides 36 distant from the ends of the slot. A minimum of 3 equally spaced slot openings on the sides 36 is desirable to insure suitable glass distribution.

The slots also contribute to the prevention of rocking and circular movement by the center electrode. A single slot in the center electrode head when filled with glass bonded to the insulator walls will prevent the center electrode from being rocked on tilted in a direction parallel to the slot as well as preventing the center electrode from being turned in a circular movement. A single slot will not prevent a rocking movement in the direction perpendicular to the slot however. For this reason, a minimum of 3 equally spaced slot openings on the sides 36 is desired to prevent a rocking movement of the center electrode in any direction. Whereas a minimum of 3 equally spaced slot openings on the sides 36 is desired for suitable glass distribution as well as preventing the rocking movement of this center electrode, a cross slot having 4 equally spaced slot openings is used in the preferred embodiment because the additional slot opening provides better glass distribution and better protection against both the circular and the rocking movement of the center electrode. Slots having more than 4 equally spaced slot openings on the sides 36 may be used in this invention.

By having the outer edge 42 in close proximity to or near touching the walls of the insulator centerbore 20, the amount of glass separating the insulator from the outer edge 42 of the center electrode head is greatly reduced. It has further been observed that reducing the amount of glass between the insulator and the outer edge 42 increases the strength of the locking arrangement since the weakest part of the locking arrangment is the glass. The close proximity or near touching of the top edge 42 to the insulator wall centerbore reduces the amount of rocking movement that the center electrode can make.

The slots, therefore, provide a passageway whereby the fluid glass flows below the outer edge 42 about the tapered sides 36 to insure that the area below the outer edge 42 and between the sides 36 and the insulator centerbore walls is completely filled with glass. The glass in these slots provide a strong bond between the glass above and below the outer edge 42. The glass in the slot combines with the glass above and below the outer edge 42 to firmly grip and lock the center electrode head in place. The glass in the slots, therefore, protects the center electrode from circular movements, rocking movements, and vertical movements.

While the invention has been described in terms of a specific embodiment, it is to be understood that the scope of the invention is not limited thereby except as defined in the following claims.

What is claimed is:

1. A spark plug comprising an insulator having a stepped centerbore therethrough, a center electrode having an enlarged head at the upper end thereof and extending through said centerbore, said enlarged head having sides which form an inverted frusto conical shape thereon, said enlarged head having the top edge thereof almost touching said insulator, a fused glass mass positioned in said centerbore above and around said head forming a seal between said insulator and said head, and said enlarged head having slot passageway means adapted to permit the flow of said glass mass in the formation of said glass seal below said top edge of said enlarged head.

2. A spark plug comprising an insulator having a stepped centerbore therethrough providing a tapered ledge therein, a center electrode having an enlarged head on the upper end thereof and extending through said centerbore, said enlarged head consisting of a tapered lower portion and an upper portion, said tapered lower portion positioned on said tapered ledge, said upper portion having sides which form an inverted frusto conical shape thereon, a fused glass mass positioned in said centerbore above and around said enlarged head forming a seal between said insulator and said head, said upper portion having the top edge thereof in close proximity to said insulator, said upper portion having slot passageway openings therein adapted to permit the flow of said glass mass in the formation of said glass seal below said top edge of said upper portion, said glass in said slots being integrally connected to the glass above said top edge and the glass below said top edge to tightly lock the enlarged head and prevent movement of the center electrode in said insulator.

3. A spark plug comprising an insulator having a stepped centerbore therethrough providing a tapered ledge therein, a center electrode having an enlarged head on the upper end thereof and extending through said centerbore, said enlarged head consisting of a tapered lower portion and an upper portion, said tapered lower portion positioned on said tapered ledge, said upper portion having sides which form an inverted frusto conical shape thereon, a fused glass mass positioned in said centerbore above and around said enlarged head forming a seal between said insulator and said head, said upper portion having the top edge thereof in close proximity to said insulator, said upper portion having a cross slot passageway thereon having four slot openings in said sides adapted to permit the flow of said glass mass in the formation of said glass seal below said top edge of said upper portion, said cross slots in the form of V-grooves which are perpendicular to each other, said glass in said slots being integrally connected to the glass above said top edge and the glass below said top edge to tightly lock the enlarged head and prevent movement of the center electrode in said insulator.

4. A spark plug comprising an insulator having a stepped centerbore therethrough providing a tapered ledge therein, a center electrode having an enlarged head on the upper end thereof and extending through said centerbore, said enlarged head consisting of a tapered lower portion and an upper portion, said tapered lower portion positioned on said tapered ledge, said upper portion having sides which form an inverted frusto conical shape thereon, said sides having a reverse taper thereon forming an angle of to 80 with a line parallel to the axis of said center electrode, a fused glass mass positioned in said centerbore above and around said enlarged head forming a seal between said insulator and said head, said upper portion having the top edge thereof in close proximity to said insulator, said upper portion having slot passageway openings therein adapted to permit the flow of said glass mass in the formation of said glass seal below said top edge of said upper portion, said glass in said slots being integrally connected to the glass above said top edge and the glass below said top edge to tightly lock the enlarged head and prevent movement of said center electrode in said insulator.

5. A spark plug as indicated in claim 4 wherein said sides form an angle of 15 to 20.

6. A spark plug comprising an insulator having a stepped centerbore therethrough providing a tapered ledge therein, a center electrode having an enlarged head on the upper end thereof and extending through said centerbore, said enlarged head consisting of a tapered lower portion and an upper portion, said tapered lower portion positioned on said tapered ledge, said upper portion having sides which form an inverted frusto conical shape thereon, a fused glass mass positioned in said centerbore above and around said enlarged head forming a seal between said insulator and said head, said upper portion having the top edge thereof in close proximity to said insulator, said sides having a reverse taper thereon forming an angle of 15 to 20 with a line parallel to the axis of said center electrode, said upper portion having slot passageway openings therein adapted to permit the flow of said glass mass in the formation of said glass seal below said top edge of said upper portion, said slot openings in the form of V-grooves having an angle of 30 to said glass in said slots being integrally connected to the glass above said top edge and the glass below said top edge to tightly lock the enlarged head and prevent movement of the center electrode in said insulator.

7. A spark plug as indicated in claim 6 wherein said angle of the V-groove slot opening is 40 to 60.

8. A spark plug comprising an insulator having a stepped centerbore therethrough providing a ledge therein, a center electrode having an enlarged head on the upper end thereof and extending through said centerbore, said enlarged head consisting of a tapered lower portion and an upper portion, said tapered lower portion positioned on said tapered ledge, said upper portion having sides which form an inverted frusto conical shape thereon, said sides having a reverse taper thereon forming an angle of 15 to 20 with a line parallel to the axis of said center electrode, a fused glass mass positioned on said centerbore above and around said enlarged head forming a seal between said insulator and said head, said upper portion having the top edge thereof in close proximity to said insulator, said upper portion having a cross slot thereon having four slot passageway openings in said sides adapted to permit the flow of said glass mass in the formation of said glass seal below said top edge of said upper portion, said cross slots in the form of V-grooves which are perpendicular to each other and having an angle in the V-groove of 40 to 60, said glass in said slots being integrally connected to the glass above said top edge and the glass below said top edge to tightly lock the enlarged head and prevent movement of the said center electrode in said insulator.

9. A center electrode for use in spark plugs having an enlarged head portion thereon, said enlarged head having sides which form an inverted frusto conical shape thereon, said enlarged head having openings thereon to provide a design having superior locking ability in said spark plug.

References Cited UNITED STATES PATENTS 3,076,113 1/1963 Candelise 313-136 X 3,229,032 1/1966 Willis 313-136 X 3,235,655 2/1966 Counts et al 313l36- X JAMES W. LAWRENCE, Primary Examiner.

C. R. CAMPBELL, Examiner. 

1. A SPARK PLUG COMPRISING AN INSULATOR HAVING A STEPPED CENTERBORE THERETHROUGH, A CENTRE ELECTRODE HAVING AND ENLARGED HEAD AT THE UPPER END THEREOF AND EXTENDING THROUGH SAID CENTERBORE, SAID ENLARGED HEAD HAVING SIDES WHICH FORM AN INVERTED FRUSTO CONICAL SHAPE THEREON, SAID ENLARGED HEAD HAVING THE TOP EDGE THEREOF ALMOST TOUCHING SAID INSULATOR, A FUSED GLASS MASS POSITIONED IN SAID CENTERBORE ABOVE AND AROUND SAID HEAD FORMING A SEAL BETWEEN SAID INSULATOR AND SAID HEAD, AND SAID ENLARGED HEAD HAVING SLOT PASSAGEWAY MEANS ADAPTED 